Type: SystemInt32If parts includes MessagePreview flag, specifies
the length of the message body section (in bytes) to be downloaded in addition to the message header section; if 0, only the message header
is downloaded; if -1 or -2, the entire message is downloaded. -1 is the only option which also sets Seen flag marking the message as read.

Return Value

Type: TaskEnvelopeCollectionA task that represents the asynchronous operation.
The value of TResult parameter is EnvelopeCollection object if message envelopes were downloaded successfully; otherwise, a null reference (Nothing in Visual Basic).